A nonaqueous secondary battery typified by a lithium ion secondary battery has high power density and high energy density, and has been generally used as a power supply for a cell phone, a note-type personal computer, or the like.
In recent years, a large battery has been practically used as a power supply for electric power storage or a power supply for electric automobiles.
Further, various electrochemical capacitors that operate in accordance with a principle similar to that of an electric double layer capacitor have been developed recently. An attention has been paid to an energy storage device combining the electricity accumulation principle of a lithium ion secondary battery and that of an electric double layer capacitor, which is called lithium ion capacitor (Non-Patent Literature 1).
An increase in the size of a battery has been required, and an increase in the useful life of the battery has also been required, since it is assumed that the battery is used under severe conditions at higher temperature especially for use in an electric automobile.
As an electrolyte solution of such a nonaqueous secondary battery, a solution in which an electrolyte is dissolved in a mixed solvent of a cyclic carbonate such as ethylene carbonate and a chain carbonate such as ethylmethyl carbonate is generally used. As a lithium salt that is the electrolyte, LiPF6 is used. This is because LiPF6 has high solubility in the solvent and high ion conductivity at a wide temperature range from low temperature to high temperature, and a side reaction is unlikely to be caused on an electrode due to the wide potential window of LiPF6. However, since LiPF6 does not have sufficient thermal stability, LiPF6 is decomposed by heating or in storage for a long period. Furthermore, an unstable intermediate produced by decomposition of LiPF6 is hydrolyzed by a trace amount of water contained in the solvent and promotes decomposition of the solvent to produce lithium fluoride and hydrogen fluoride. When LiPF6 is decomposed, the ion conductivity of an electrolyte solution containing the decomposed products of LiPF6 decreases. At the same time, the produced lithium fluoride and hydrogen fluoride corrode materials for an electrode and a collector, and a gas is generated by decomposition of the solvent to increase a pressure inside the battery. The battery may be significantly adversely affected (Non-Patent Literatures 2 and 3).
As an electrolyte having higher thermal stability to overcome the disadvantages of LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, and the like are known. However, a nonaqueous electrolyte solution of LiBF4 or LiCF3SO3 has improved thermal stability as compared with LiPF6, but a problem of decrease in the ion conductivity arises. A nonaqueous electrolyte solution of LiN(CF3SO2)2 does not have sufficient oxidation resistance, and has a problem such as corrosion of aluminum metal that is used for a positive electrode collector.
On the other hand, Patent Literatures 1, 2, and 3 disclose a method for increasing thermal stability and hydrolysis resistance of an electrolyte by using fluoroalkyl phosphate in which fluorine atoms of LiPF6 are partially substituted by a perfluoroalkyl group. However, this prior art also has a problem of decrease in oxidation resistance since fluorine atoms bonded to a phosphorous atom are partially substituted with carbon atoms.
Non-Patent Literatures 4 and 5 and Patent Literatures 4 and 5 propose a specific phosphorous acid ester(tris(2,2,2-trifluoroethyl)phosphite) and hexamethylphosphoramide as an additive that thermally stabilizes LiPF6. However, the phosphorous acid ester and hexamethylphosphoramide herein are not electrochemically stable. Therefore, when a battery containing a phosphorous acid ester or hexamethylphosphoramide is repeatedly charged and discharged at high voltage, decomposition occurs, and the battery performance is insufficient.
As described above, LiPF6 having high ion conductivity and wide potential window is an essential electrolyte to exhibit good battery performance, but has a problem of thermal stability. Even when a specific phosphorous acid ester or hexamethylphosphoramide proposed to solve the problems is added, acceptable battery performance cannot be obtained due to poor electrochemical stability.